Odd powers of 2 written between a pair of 1's.

Terms

a(0) =121a(1) =181a(2) =1321a(3) =11281a(4) =15121a(5) =120481a(6) =181921a(7) =1327681a(8) =11310721a(9) =15242881a(10) =120971521a(11) =183886081a(12) =1335544321a(13) =11342177281a(14) =15368709121a(15) =121474836481a(16) =185899345921